A conventional process for separating hydrogen and carbon monoxide comprises a low temperature scrubbing step using liquid methane to dissolve carbon monoxide, a hydrogen stripping column or flash separator to separate hydrogen from the CO-loaded methane (containing about 3%-4% H.sub.2), and a carbon monoxide/methane separation column to separate the hydrogen-stripped CO-loaded methane into carbon monoxide and methane fractions. The hydrogen stripping column normally operates at an intermediate pressure to the methane wash and carbon monoxide/methane separation columns.
GB-A-1,579,553 discloses a said conventional process in which the CO-loaded methane is countercurrently contacted with hydrogen-rich vapour generated by warming the hydrogen-stripped CO-loaded methane internally or externally of the hydrogen stripping column.
U.S. Pat. No. 5,295,356 discloses a said conventional process in which the columns are heated and/or cooled by a closed refrigerating cycle, preferably a nitrogen cycle.
U.S. Pat. No.5,592,831 discloses a said conventional process in which the methane wash is conducted with methane containing at least 2 to 15 mol % carbon monoxide. In the exemplified embodiment, the methane scrubbing column and hydrogen stripper are combined into a single column and the hydrogen-stripped CO-loaded methane is fed to the carbon monoxide/methane separation column partly as a subcooled liquid and the remainder after vaporization.
A major problem with the conventional process is that a significant amount of carbon monoxide is lost in the overhead ("reject hydrogen") stream from the hydrogen stripping column or flash separator. This problem has been solved in the past by recycling the reject hydrogen stream to recover the carbon monoxide (and hydrogen), or by washing the reject hydrogen stream with essentially pure liquid methane recycled from the sump of the carbon monoxide/methane separation column to dissolve and recover most of the carbon monoxide (as described in U.S. Pat. No. 4,888,035).